A Key to the More Frequently Occurring Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 236

(20-50 μm long, 5-12 μm wide) contain
a single chloroplast with central pyrenoid.
A widespread species occurring on damp
surfaces such as rocks, stones or soil.
Bacillariophyta. Plate XXVI.

198 (196) (a) Valve surface covered by fine striae
with irregular gaps. Cells lanceolate,
some with capitate/rostrate apices. Valve
face with thickened ridge along edge
........................... Brachysira

Brachysira cells are solitary. The genus
is widespread in many waters but espe-
cially in flowing systems. Raphe straight.
Cells (14-115 μm long, 4-21 μm wide)
have a single chloroplast. Bacillariophyta.
Plate XXVI.

100 μm

Figure 4.71 Pinnularia . Valve view of this large ben-
thic diatom. Prepared slide.

(b) Valvesnotasabove............ 199

199 (198) (a) Striae parallel to raphe and cross-
hatched. Very small clear central area
which may be completely absent. Valves
widely lanceolate to slightly elliptical.
Two plate-like chloroplasts . . . . Craticula

Pinnularia is a large genus. Cells linear,
lanceolate or even elliptical. The poles
are usually rounded, capitate or rostrate.
Striae are usually coarse (but may be
finer in some species). There is a cen-
tral raphe whose middle ends bend in the
same direction. Cells 20-200 μm long,
4-50 μm wide. Usually two plate-like
chloroplasts, one either side of the mid-
line. Some species may have other shaped
chloroplasts. Widespread and common on
sediments and other substrata and may
be mixed in with moss clumps. Water
types range from nutrient-poor to nutrient-
rich and slightly alkaline to mildly acidic.
Bacillariophyta. Plate XXVII. Figs. 1.13
and 4.71.

The genus Navicula is a very large
genus the species of which are sometimes
hard to distinguish. With the advent of
new techniques such as scanning electron
microscopy several smaller genera have
been divided off. These include Craticula ,
Diadesmis and Luticola.

Craticula cells (40-150 μm long, 13-
30 μm wide) have two plate-like chloro-
plasts and two large obvious droplets.
Common in a range of electrolyte-rich
waters, both fresh and brackish. Bacillar-
iophyta. Plate XXVI.

(b) Striae finer and not rib-like...... 201

(b) Clear central area present, striae not
cross hatched. ..................... 200

201 (200) (a) Raphe lying within a thickened ridge
.................................. 202

200 (199) (a) Striae rib-like in appearance
........................... Pinnularia

(b) Raphe not lying within a thickened
ridge........................ Navicula